Combination process for the conversion of heavy distillates to LPG

ABSTRACT

Maximum conversion of heavy distillates to LPG is achieved through a combination process involving two stage hydrocracking.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of my prior copendingapplication Ser. No. 489,014 filed July 16, 1974, now U.S. Pat. No.3,917,562. All of the teachings of this prior application arespecifically incorporated herein by reference.

The invention encompassed by the present application relates to a twostage hydrogen-consuming process for selectively producing LPG from ahydrocarbon charge stock boiling above 600°F. Hydrocracking processesare commonly employed for the conversion of heavier hydrocarbons intolower boiling saturated products. Economically successful LPGhydrocracking processes must be selective in order to avoid thedecomposition of normally liquid hydrocarbons into undesirable gaseoushydrocarbons such as methane and ethane while maintaining a highactivity for extended periods of time. The present invention utilizes acatalytic composite comprising a porous carrier material, a Group VI-Bmetal component and a Group VIII metal component in the first stagehydrocracking zone and utilizes a catalytic composite comprising a GroupVIII metal component combined with a support containing alumina andfinely divided mordenite particles in the second stage hydrocrackingzone.

Solid catalysts having a propensity to accelerate so-calledacid-catalyzed reactions are widely used today in hydrocrackingprocesses. In many applications these catalysts are used by themselvesto accelerate the reactions which mechanically are thought to proceed bycarbonium ion intermediates. In other applications these acidiccatalysts are combined with a hydrogenation-dehydrogenation metalliccomponent to form a dual-function catalyst having both a crackingfunction and a hydrogenation-dehydrogenation function. In this lattercase, the cracking function is generally thought to be associated withan acid-acting material of the porous adsorptive, refractory oxide-typewhich is typically utilized as the support or carrier for a heavy metalcomponent such as the metals or compounds of metals of Group VI or GroupVIII of the Periodic Table to which the hydrogenation-dehydrogenationfunction is generally attributed.

In order to effect an acceptable, economically feasible hydrocrackingprocess, the prior art proposes to combine crystalline aluminosilicateswith an alumina material to produce a catalyst having an acidic functionwhich is substantially greater than the sum of the acidity contributedby the alumina alone and the crystalline aluminosilicate alone.

The primary objective of the present invention is to provide animprovement in the process for selectively producing LPG from ahydrocarbon charge stock boiling above 600°F. As hereinafter indicatedby specific example, the improvement resides in the chemical characterof the catalytic composites which may be used in the second stagecatalytic reaction zone. The use of the improvement of the presentinvention results in a process which exhibits an increased activitywithout sacrificing the selectivity of the catalyst to product LPG. Arelated object is, therefore, to provide a process which functionseconomically for an extended period of time as a result of the increasedefficiency arising through the use of the improved catalytic composite.

Therefore, in a broad embodiment, the present invention relates to animprovement in a hydrogen-consuming process for selectively producingLPG from a hydrocarbon charge stock boiling above about 600°F.,contacting said charge stock and hydrogen with a catalytic compositecomprising a porous carrier material, a Group VI-B metal component and aGroup VIII metal component in a first catalytic reaction zone athydrocracking conditions and then reacting at least a portion of thefirst catalytic reaction zone effluent with a catalyst comprising aGroup VIII metal component and combined with an alumina carrier materialcontaining a uniform distribution of finely divided mordenite particlesin a second catalytic reaction zone, at hydrocracking conditionsincluding a pressure of about 300 to about 1800 psig., a temperature ofabout 600°F. to about 850°F., an LHSV of about 1 to about 10 hr..sup.⁻¹and a hydrogen circulation rate of about 5000 SCFB to about 15,000 SCFBbased on fresh charge stock, which catalyst is prepared by comminglingsaid mordenite with an aluminum halide sol, gelling the resultantmixture, then calcining the gelled mixture, wherein said alumina carriermaterial contains about 20 to about 30 weight percent mordenite.

As hereinbefore set forth, the process of the present invention isparticularly directed to the processing of hydrocarbons and mixtures ofhydrocarbons boiling above 600°F. Since the production of LPG is to bemaximized, suitable charge stocks will include heavy distillatehydrocarbons having an initial boiling point of about 600°F. to about1000°F. and an end boiling point which may range from about 650°F. toabout 1050°F. These charge stocks may be isolated by well-knownprocessing techniques from tar sand, shale and coal. The effluentstreams from cokers, thermal crackers, fluid catalytic crackers, crudeunits and visbreakers may also supply distillate hydrocarbons for thesecharge stocks.

Such charge stocks may be successfully processed even though quantitiesof sulfur are present. However, for best results, the hydrocarbon feedto be utilized in the second stage preferably contains less than about50 ppm. sulfur and more preferably less than 10 ppm. sulfur.

As hereinabove described, the catalyst utilized in the first stagecomprises a porous carrier material, a Group VI-B metal component and aGroup VIII metal component. This catalyst may also contain a halogencomponent, preferably chlorine or fluorine and a sulfur component. Thiscatalyst is commercially available and the prior art abounds withmethods for its preparation.

As indicated above, the catalyst for the second stage comprises a GroupVIII metal component combined with a support containing alumina andmordenite particles. Considering first the alumina, it is preferred thatthe alumina be a porous, adsorptive, high surface area material having asurface area of about 25 to about 500 or more square meters per gram.Suitable alumina materials are the crystalline aluminas known as gamma-,eta-, and theta-alumina with gamma-alumina giving best results.

It is an essential feature of the present invention that the aluminasupport contains finely divided mordenite particles. As is well known tothose skilled in the art, mordenite is composed of a three-dimensionsalinterconnecting network structure of silica and alumina tetrahedra. Thetetrahedra are formed by four oxygen atoms surrounding a silicon oraluminum atom, and basic linkage between the tetrahedra are through theoxygen atoms. These tetrahedra are arranged in an ordered structure toform interconnecting cavities or channels of uniform size interconnectedby uniform openings or pores. The ion-exchange property of mordenitefollows from the trivalent nature of aluminum which causes the aluminatetrahedra to be negatively charged and allows the association ofcations with them in order to maintain an electrical balance in thestructure. The molecular sieve property of mordenite follows from theuniform size of the pores thereof which pores can be related to the sizeof molecules and used to remove a mixture of molecules, the moleculeshaving a critical diameter less than or equal to the diameter of thepore mouths. For purposes of the present invention, it is preferred touse mordenite having pore mouths of about 5 Angstroms in cross-sectionaldiameter and more preferable about 5 to about 15 Angstrom units.Ordinarily, mordenite is synthetically prepared in the alkali metal formwith one alkali metal cation associated with each aluminum centeredtetrahedra. The alkali metal cation may be thereafter ion-exchanged withpolyvalent cations such as calcium, magnesium, beryllium, rare earthcations, etc. Another treatment of alkali metal mordenite involvesion-exchange with ammonium ions followed by thermal treatment,preferably above 300°F. to convert to the hydrogen form. When themordenite contains a high mole ratio of silica to alumina (for example,above 5) the material may be directly converted to an acid form in asuitable acid medium.

Although in some cases the polyvalent form of the mordenite may be usedin the present invention, it is preferred to use the hydrogen form suchas the alkali metal form, which is convertible to the hydrogen formduring the course of the essential incorporation procedure discussedbelow.

The preferred mordenite for use in the present invention is the hydrogenand/or polyvalent forms of synthetically prepared mordenite. In fact, Ihave found best results with synthetic mordenite having an effectivepore diameter of about 4 to about 7 Angstrom units and a mole ratio ofsilica to alumina of about 10 to about 25, preferably from about 11 toabout 16. As is well known to those skilled in the art, mordenitediffers from other known crystalline aluminosilicates in that itscrystal structure is believed to be made up of chains of 5-member ringsof tetrahedra which apparently are arranged to form a parallel system ofchannels having diameters of about 4 to 7 Angstroms interconnected bysmaller channels having a diameter of about 2.8 Angstroms. Mordenite ischaracterized by the following formula:

    0.9 ± 2M.sub.2/n O : Al.sub.2 O.sub.3 : X SiO.sub.2 (anhydrous form)

wherein M is a cation which balances the electrovalences of thetetrahedra, n is the valence of M, and X is a constant generally rangingin value from 9 to 11 and usually about 10. The synthetic mordenite typezeolites are available from a number of sources, one being the NortonCompany of Worcester, Mass.

Regarding the method of incorporating the mordenite particles into thealumina support, it is an essential feature of the present inventionthat the mordenite particles are added directly to an alumina hydroxylhalide sol prior to the sol being gelled. Although in some cases solformed with fluorine, bromine, or iodine, may be satisfactory, I havefound best results are obtained with an aluminum hydroxyl chloride solformed by dissolving substantially pure aluminum metal in hydrochloricacid to result in a sol having a weight ratio of aluminum to chloride ofabout 1:1 to about 1.4:1. Additionally, it is preferred that the solhave a pH of about 2 to about 5. One advantage of this feature of thepresent invention is the relative ease with which the mordeniteparticles can be uniformly distributed in the resulting catalyst.However, the most important advantage is that the sol appears to reactwith the mordenite, causing some basic modification of its structurewhich enables the resulting support to have unusual ability to catalyzereactions which depend on carbonium ion intermediates such ashydrocracking to C₃ and C₄ fragments.

Accordingly, it is an essential feature of the present invention thatthe second stage catalyst is produced by the following steps:commingling finely divided mordenite particles with an aluminum hydroxylhalide sol to form a mixture thereof; gelling the resultant mixture toproduce a hydrogel or particles of a hydrogel; then finishing thehydrogel into the catalyst by standard aging, washing, drying, andcalcination treatments. For purposes of the present invention, thecatalyst may be formed in any desired shape such as spheres, pellets,pills, cakes, extrudates, powders, granules, etc. However a particularlypreferred form of the catalyst is the sphere; and spheres may becontinuously manufactured by the well known oil drop method whichcomprises forming an alumina hydrosol, preferably by reacting aluminummetal with hydrochloric acid, combining the hydrosol with a suitablegelling agent such as hexamethylenetetramine to form a droppingsolution, uniformly distributing finely divided mordenite particlesthroughout the dropping solution, and dropping the resultant mixturethereof and the gelling agent thereafter added to the mixture to formthe dropping solution. In either case, the droplets of the mixtureremain in the oil column until they set and form substantially sphericalhydrogel particles. The spheres are then continuously subjected tospecific aging treatments in oil and an ammoniacal solution to furtherimprove their physical characteristics.

Alternatively, the hydrogel spheres may be pressure aged in the droppingoil or a similar oil which may make atmospheric aging in an ammoniacalsolution unnecessary. Suitable conditions for pressure aging wouldinclude a temperature from about 20°C. to about 300°C. with a pressuresufficient to maintain the system in liquid phase. The resulting agedand gelled particles are then washed and dried at a reltively lowtemperature of about 300°F. to about 400°F. and subjected to acalcination procedure at a temperature of about 850°F. to about 1300°F.for a period of about 1 to about 20 hours. This treatment effectsconversion of the alumina hydrogel to the corresponding crystallinegamma-alumina. See U.S. Pat. No. 2,620,314 for additional detailsregarding this oil drop method.

The amount of mordenite in the resulting alumina support is preferablyabout 20 to about 30 weight percent thereof. By the expression "finelydivided" it is meant that the mordenite is used in a particle sizehaving an average diameter of about 1 to about 100 microns, with bestresults obtained with particles of average diameter or less than 40microns.

The catalyst for the second stage may contain a halogen component.Although the precise form of the chemistry of this association of thehalogen component with the alumina support is not entirely known, it iscustomary in the art to refer to the halogen component as being combinedwith the alumina support, or with the other ingredients of the catalyst.This combined halogen may be either fluorine, chlorine, iodine, bromine,or mixtures thereof. Of these, fluorine, and particularly chlorine arepreferred for the purposes of the present invention. As indicated above,a halogen component is inherently incorporated in the catalyst duringpreparation thereof. If desired, additional halogen may be added to thecalcined catalyst as an aqueous solution of an acid such as hydrogenfluoride, hydrogen chloride, hydrogen bromide, etc. Moreover, anadditional amount of the halogen component, may be composited with thecatalyst during the impregnation of the latter with the Group VIII metalcomponent. In any event, the halogen compound may be combined with thesupport in amounts sufficient to result in a final catalyst whichcontains about 0.01 to about 1.5 weight percent and preferably about 0.1to about 1.0 weight percent halogen calculated on an elemental basis.

An essential component of the second stage catalyst is the Group VIIImetal component. The Group VIII metal may exist within the finalcatalytic composite as a compound such as an oxide, sulfide, halide, orin an elemental state. Generally the amount of the Group VIII metalcomponent present in the final catalyst is small compared to the othercomponents combined therewith. The Group VIII metal component generallycomprises about 0.05 to about 1.5 weight percent of the final catalyticcomposite calculated on an elemental basis. Suitable Group VIII metalsare platinum, iridium, osmium, palladium, rhodium, ruthenium, nickel,cobalt and iron. However, palladium and platinum are preferred.

The Group VIII metal component may be incorporated in the second stagecatalytic composite in any suitable manner as ion-exchange and/orimpregnation with a suitable solution of the metallic component.However, it is an essential feature of the present invention that theGroup VIII metal component is combined with the catalyst base preparedby the method of the present invention after the calcination stepdescribed above. Accordingly, the preferred method of preparing adual-function catalyst comprising a Group VIII metal component combinedwith the catalyst prepared by the method outlined above involves theutilization of water-soluble compounds of the Group VIII metal componentto impregnate the calcined catalyst. For example, platinum metal may beadded to the support by commingling the latter with an aqueous solutionof chloroplatinic acid.

Regardless of the details of how the Group VIII metal component of thecatalyst is combined with the catalyst, the resulting dual-functioncatalyst generally will be dried at a temperature of from about 200°F.to about 600°F. for a period of from about 2 to 24 hours or more andfinally calcined at a temperature of about 700°F. to about 1100°F. for aperiod of about 0.5 to about 10 hours, and preferably 1 to about 5hours.

It is preferred that the resultant calcined dual-function catalyticcomposite be subjected to reduction conditions prior to its use in theconversion of hydrocarbons. This step is designed to insure a uniformand finely divided dispersion of the Group VIII metal componentthroughout the carrier material. Preferably, substantially pure and dryhydrogen is used as the reducing agent in this step. The reducing agentis contacted with the calcined catalyst at a temperature of about 800°F.to about 1200°F. and a period of time of about 0.5 to 10 hours or moreeffective to substantially reduce the Group VIII metal component to itselemental state. This reduction treatment may be performed in situ aspart of a start-up sequence if desired.

Althouggh it is not essential, the resulting reduced dual-functioncatalyst is preferably subjected to a presulfiding operation designed toincorporate in the catalytic composite from about 0.05 to about 1.5weight percent sulfur calculated on an elemental basis. Preferably, thispresulfiding treatment takes place in the presence of hydrogen and asuitable sulfur-containing compound such as hydrogen sulfide, lowermolecular weight mercaptans, organic sulfides, etc. Typically, thisprocedure comprises treating the reduced catalyst with a sulfiding gassuch as a mixture of hydrogen and hydrogen sulfide having about 10 molesof hydrogen per mole of hydrogen sulfide at conditions sufficient toeffect the desired incorporation of the sulfur component, generallyincluding a temperature ranging from about 50°F. to about 1100°F. ormore.

Both reduction and presulfiding of the second stage catalyst mayalternatively be performed simultaneously by contacting the calcinedcatalyst with a gas such as a mixture of hydrogen and hydrogen sulfidehaving about 10 moles of hydrogen per mole of hydrogen sulfide atconditions sufficient to effect the desired reduction and sulfiding,generally including a temperature ranging from about 50°F. to about1100°F. or more.

According to the present invention, a hydrocarbon is contacted with thecatalysts of the types described above in a first and second hydrocarbonconversion zone at hydrocarbon conversion conditions. This contactingmay be accomplished by using the catalysts in a fixed bed system, amoving bed system, a fluidized bed system, or in a batch type operation;however, in view of the danger of attrition losses of the valuablecatalysts and of well known operational advantages, it is preferred touse fixed bed systems. In these systems, the charge stock is preheatedby any suitable heating means to the desired reaction temperature andthen passed into a conversion zone containing a fixed bed of catalyst.It is, of course, understood that either the first or second conversionzone may be one or more separate reactors with suitable meanstherebetween to insure that the desired conversion temperature ismaintained at the entrance to each reactor. It is also to be noted thatthe reactant may be contacted with the catalyst bed in either upward,downward, or radial flow fashion. In addition, it is to be noted thatthe reactants may be in the liquid phase, a mixed liquid-vapor phase, ora vapor phase when they contact the catalyst.

A heavy distillate hydrocarbon is charged to the first stage reactionzone which is maintained at hydrocracking conditions. An effluent streamcontaining hydrocarbons having a lower boiling range than the feedstockis withdrawn from the first stage conversion zone and which is strippedof hydrocarbon light ends comprising LPG and any other normally gaseouscompounds, such as ammonia and hydrogen sulfide. At least a portion ofthe stripped product is introduced into the second stage conversion zonewhich is maintained at hydrocracking conditions. An effluent stream iswithdrawn from the second stage conversion zone and passed through acondensing means to a separation zone, typically maintained at about 50°to about 125°F., wherein a hydrogen-rich gas is separated from a LPGrich liquid product. Preferably, at least a portion of thishydrogen-rich gas is withdrawn from the separating zone and is thenrecycled through suitable compressing means back to the second stageconversion zone. The liquid phase from the separating zone is thentypically withdrawn and commonly treated in a fractionating system inorder to recover LPG (i.e., liquefied petroleum gas) and other lightends.

The following example is given to further illustrate the process of thepresent invention and to indicate the benefits to be afforded throughthe utilization thereof. It is understood that the example is given forthe sole purpose of illustrating a method for the practice of thepresent invention and that the example is not intended to limit thegenerally broad scope and spirit of the appended claims.

EXAMPLE

A diesel oil having a boiling range of 596°F. to 654°F. and a sulfurcontent of 1.0 weight percent was selected for maximum conversion toLPG. This diesel oil was processed in a hydrocracking reaction zonecontaining a catalyst comprising nickel and molybdenum supported to asilica-alumina carrier material, at a pressure of 1500 psig., a liquidhourly space velocity (LHSV) of 0.8 hr..sup.⁻¹, and a hydrogencirculation rate of 12,000 standard cubic feet per barrel (SCFB) toyield a 550°F. end point kerosene containing only 3 ppm. sulfur. Thiskerosene was stripped to remove hydrocarbon light ends and hydrogensulfide and then was processed in a second hydrocracking reaction zonecontaining a catalyst comprising platinum, alumina and mordenite andprepared as hereinabove described, at a pressure of 1000 psig., a liquidhourly space velocity of 3.0 hr..sup.⁻¹ and a hydrogen circulation rateof 10,000 SCFB to yield 75 weight percent LPG and 25 weight percentpentanes.

The foregoing specification and example clearly illustrate theimprovements encompassed by the present invention and the benefits to beafforded a process for the production of LPG from higher-boilinghydrocarbon charge stock.

I claim as my invention:
 1. A process for the conversion ofsulfur-containing, heavy distillate hydrocarbons boiling above about600°F. to produce LPG, which process comprises the steps of:a. reactingsaid sulfur-containing, heavy distillate in a first reaction zonecontaining a catalytic composite comprising a porous carrier material, aGroup VI-B metal component and a Group VIII metal component athydrocracking conditions selected to convert the heavy distillate intolower boiling hydrocarbons; b. reacting at least a portion of said lowerboiling hydrocarbons containing less than about 50 ppm. sulfur in asecond reaction zone containing a catalytic composite comprising a GroupVIII metal component combined with an alumina carrier materialcontaining a uniform distribution of finely divided mordenite particleswhich catalytic composite is prepared by commingling said mordenite withan aluminum halide sol, gelling the resultant mixture then calcining thegelled mixture wherein said alumina carrier material contains from about20 to about 30 weight percent mordenite, at hydrocracking conditionsselected to produce LPG; and, c. recovering said LPG from the resultingfirst and second reaction zone effluents.
 2. The process of claim 1further characterized in that said lower boiling hydrocarbons of step(b) contain less than 10 ppm. sulfur.
 3. The process of claim 1 furthercharacterized in that said sulfur-containing, heavy distillate is dieseloil or gas oil.
 4. The process of claim 1 further characterized in thatsaid catalytic composite of said first reaction zone comprises analuminosilicate carrier material, a nickel component and a molybdenumcomponent.
 5. The process of claim 1 further characterized in that saidhydrocracking conditions include a pressure from about 300 to about 2000psig., a temperature from about 600°F. to about 850°F., a LHSV of about0.5 to about 10 hr..sup.⁻¹ and a hydrogen circulation rate of about 5000SCFB to about 15,000 SCFB.
 6. The process of claim 1 furthercharacterized in that said catalytic composite of said second reactionzone comprises platinum or a platinum compound.
 7. The process of claim1 further characterized in that said catalytic composite of said secondreaction zone comprises palladium or a palladium compound.
 8. Theprocess of claim 1 further characterized in that said catalyticcomposite of said second reaction zone contains about 0.05 to about 1.5weight percent halogen.
 9. The process of claim 1 further characterizedin that said catalytic composite of said second reaction zone containsabout 0.1 to about 1.5 weight percent sulfur.